What Is a Reverse Osmosis Plant?

Understanding reverse osmosis requires first understanding osmosis. And understanding osmosis requires understanding diffusion. These concepts are not as daunting as they might seem and once established can be very useful in a variety of applications. A reverse osmosis plant can be small or large, but no matter what the scale, it features several key components necessary to carry out the process.

Diffusion

Draw a glass of warm tap water and drop in a pinch or two of salt (sodium chloride). Even without stirring, the salt will begin to dissolve in the warm water. Sodium atoms and chlorine atoms separate from the structure of the solid salt grains and mix with water molecules. The sodium and chlorine are in a charged condition. Sodium is positive and chlorine negative, and in this condition, strictly speaking, they are called ions and not atoms. Sodium is in the form of the sodium ion and chlorine is in the form of the chloride ion. In this separated, charged state the ions tend to disperse evenly throughout the glass of water.

Osmosis

Osmosis is diffusion through a membrane. Say the glass in the diffusion example has a membrane dividing the inside into two halves. Put water up to the same level in each side, say about half way. Now drop a pinch of salt in each side, except put a larger pinch in one side. The salt dissolves and diffuses into the water as in the explanation of diffusion. But this time things are a bit different. The glass is divided in two, and even though there's the same amount of water in each side there's more salt in one side than the other.
Membranes can be what is called semi-permeable. This means that the membrane will allow some things to pass through but not others. In this case the membrane will allow water to pass through but not the salt ions. As in the diffusion example, the natural tendency is for the solution to become evenly concentrated throughout. This time, however, the membrane is blocking the way for the salt ions. But since the water can move through, it does.
Water passes through the membrane from the side containing the lower concentration of salt to the side with the higher concentration. This process achieves uniform salt concentration between the two sides. The liquid level becomes higher on the side where the larger pinch of salt was placed but the result is that each side is equally as salty. This is osmosis.

Reverse Osmosis

What if---instead of letting osmosis take its natural course and even out the salt concentration in a solution---you want to remove all the salt from the solution on one side of the membrane? It turns out that forcing osmosis to operate in reverse can accomplish this. Applying pressure to the solution on one side of the membrane can force the water through in the opposite direction from its natural tendency. Continued long enough, this will result in pure water on one side and more and more concentrated salt solution on the other. The benefits are fairly obvious. This is a way of purifying water containing salt or other impurities.

Industrial Applications

Many industrial applications employ reverse osmosis. Among the examples are the making of maple syrup and the desalinization of sea water. In the medical industry, procedures like kidney dialysis are based on the principle of reverse osmosis.

Science Applications

Laboratories around the world rely on purified water for chemical analyses and other procedures. Reverse osmosis is one of the methods of supplying this water.